1. Field of the Invention
The present invention relates to an image signal scrambling apparatus and a descrambling apparatus, and more particularly to an image signal scrambling apparatus and a descrambling apparatus applied to an image signal coding apparatus used for a broadcasting satellite and a communication satellite, which utilizes a discrete cosine transform (DCT) coding method and a variable-length-coding method.
2. Description of the Prior Art
Up to now, as shown in FIG. 2, a system for transmitting and receiving an image signal coded by means of DCT coding and variable-length-coding comprises a transmitting apparatus 101, a receiving apparatus 102, and a satellite communication path 3. The transmitting apparatus 101 includes an A/D converter 11 for converting an analog image signal V1 to a digital image signal V2, a DCT encoder 12 for encoding the converted digital image signal V2 and outputting encoded signals of a scalar quantumizing number D1, a block type D2, a motion compensation vector D3, effective data D4, and the number of zero-runs D5, a variable length-encoder 13 for variable-length-encoding the encoded signals D2, D3, D4 and D5 (except the scalar quantumizing number D1) by means of a Huffmann code system, a framing circuit 14 for framing a variable-length-encoded image signal V3 and a scalar quantumizing number D1 to form a scrambled image signal v4, a buffer memory 15 for temporarily storing the scrambled image signal V4' composed by the framing circuit 14 and permitting a change in the processing speed by controlling the amount of information generated, and another framing circuit 116 for framing the image signal V4' from the buffer memory 15 and voice data A into a composite image signal D7' and outputting the composite signal D7' onto a transmission path. The image signal, compressed by means of DCT encoding and variable-length-encoding in the transmitting apparatus 101, is transmitted to the receiving apparatus 102 through a satellite path 3.
The receiving apparatus 102 includes a deframing circuit 121 for separating the composite signal D7' into the image signal V4' and voice data A, a buffer memory 22 for temporarily storing the image signal V4', permitting a change in processing speed, another deframing circuit 23 for separating the image signal V4' stored in the buffer memory 22 into a scalar quantumizing number D1 and a variable-length-encoded image signal V3 and outputting those separated signals, a variable-length decoder 24 for giving a block type D2, a motion compensation vector D3, effective data D4, and the number of zero-runs D5 by variable-length-decoding the variable-length-encoded image signal V3 by means of the Huffmann code system, a DCT decoder 25 for making those variable-length-decoded data D2, D3, D4 and D5 and the scalar quantumizing number D1 into a digital image signal V2' through run-length, scalar-quantumizing, predictive-transform, and DCT decoding processes, and a D/A converter 26 for converting the DCT-decoded digital image signal V2' into an analog image signal V1'.
Since the above-described transmitting apparatus and receiving apparatus of coded digital image signals using DCT encoded and variable-length-encoding as described above do not have a signal encrypting function, it is not possible to make a communication service chargeable or provide an encrypted communication in a cable transmission service, a satellite broadcasting service, and the like.